Periodic or random vibrations or shocks can excite the induction core of a stator, rotor, or transformer to vibrate at its resonant frequencies. These can be problematic due to the resultant formation of undesirable stresses, displacements, fatigue, and sound radiation. Such undesirable vibrations or shocks are typically induced by the interaction of the stator and rotor. The resulting magnetic flux interactions of the stator and rotor can lead to vibration in either of the components.
Flux build-up and flux fields interacting with each other at high rates can lead to a flux field force being applied to the stator or rotor, causing it to vibrate at resonant frequencies resulting in sound radiation, fatigue, and vibrations that can be transmitted to other portions of the motor or items attached to the motor leading to degraded performance (such as in a disk drive where the excessive vibrations transmitted by the stator can cause read and write errors and reduced drive performance) and/or excessive heat build-up from frictional movement. A quiet motor is important to many applications. For example, disk drives often require a motor to operate at less than 30-45 dBA so that when the drive is used in an application, it does not bother the end user of the drive.
Typical stators, rotors, and transformers have many magnetic layers. These layers are stamped or otherwise cut in single individual layers, typically a ring pattern with "poles" extending from the ring. The magnetic layers are stacked by hand or by automated assembly to the desired stack height of the stator, rotor, or transformer. The layers are then joined in a number of ways, such as, for example, by pressing the layers together with a die and using a punch to crimp the layers together, coining, embossing, encasing or coating the outer surfaces of the assembled layers with a rigid non-vibration damping polymeric material (such as an epoxy), applying heat and pressure to form a hydrostatic bond, or combinations of the aforementioned.
Various techniques have been used to reduce vibrational and shock effects (stresses, displacements, etc.) on stators. Three basic techniques include:
1) adding stiffness or mass to the stator so that the resonant modes of the stator are not excited in operation; PA1 2) isolating the stator so that the vibrational or shock energy does not excite other items in the motor construction or items connected to the motor; and PA1 3) damping the stator core by "potting" or encasing the stator core exterior, or portions of the stator core exterior in a polymeric potting material. Typically a polymeric potting material, which optionally may have some damping benefit and/or stiffness benefit, may be used to encase all or a portion of the stator core exterior, thereby reducing the vibration excitation levels and the harmful effects. PA1 (a) two outer magnetic layers; PA1 (b) an inner layer of vibration damping material comprising a viscoelastic material positioned between the two outer magnetic layers; wherein the core is selected from the group consisting of stator, rotor and transformer cores. PA1 (a) two outer magnetic layers; PA1 (b) at least two inner vibration damping material layers comprising a viscoelastic material positioned between the two outer magnetic layers; PA1 (c) at least one magnetic layer positioned interior to the outer magnetic layers; PA1 wherein the core is selected from the group consisting of stator, rotor, and transformer cores. PA1 (a) providing a first laminate section and a second laminate section, wherein the first laminate section is formed by a method comprising the steps of PA1 wherein the second laminate section is formed by a method comprising the steps of PA1 (b) joining the first laminate section and the second laminate section to form a core in such a manner that the burr is not on the outside of the core; PA1 (a) providing at least two laminate sections, wherein each laminate section is independently selected from the group consisting of first laminate sections and second laminate sections; and PA1 wherein each first laminate section is independently formed by a method comprising the steps of PA1 wherein each second laminate section is independently formed by a method comprising the steps of PA1 (b) joining the laminate sections together to form a core in such a manner that no exposed burr is present on the core;
Katakura et al., U.S. Pat. No. 5,241,229 discloses a magnetic disc drive motor comprising a hub for carrying magnetic discs on its outer periphery, a drive magnet rigidly fitted to the inner periphery of the hub, a stator core having a coil wound around it and juxtaposed with the drive magnet, and a motor frame having a substantially cylindrical holder for rigidly holding the stator core, wherein the space between the stator core and the motor frame is filled with a resin material. A compact and simply configured magnetic disc drive motor can thus be realized, which is capable of rotating magnetic discs in a very stabilized manner. With such an arrangement, since the space between the stator core and the motor frame, which was not used for any particular purpose, is filled with a resin material, the space within the motor is effectively utilized to enhance the rigidity of the motor frame.
The aforementioned design provides for a stiffer stator thereby reducing some vibration levels. In addition, the resin material can also provide some vibration damping. This design, however, is difficult to manufacture as filling between the poles is difficult. The design can also reduce the heat flow away from the stator increasing the temperature of the windings and creating various problems related to high temperatures of the motor.
This design would also be expensive to manufacture as each stator with windings in place would need to be potted, requiring fixturing, cure ovens, and potential long manufacturing cycles. Variability between stators being potted would be expected as each would have variations due to tolerances of the windings, etc. The potted design would also require a significant amount of organic resin material to be used inside the motor and disk drive assembly. The organic resin material would be largely exposed to the internal motor and drive atmosphere which could lead to outgassing concerns when the drive operates at elevated temperatures (typically greater than 45.degree. C., most typically greater than 60.degree. C.). The outgassing could lead to corrosion or outgassed material build-up on various parts inside the motor or drive, such as on a disk, read/write heads and other exposed surfaces leading to drive performance reductions or drive failure.
Von Der Heide, et al., U.S. Pat. No. 4,647,803 discloses an electric motor with a substantially cylindrical air gap between the stator and the rotor, the stator being fitted to a bearing support for the rotor shaft bearing. In order to reduce noise emissions, the stator is connected to the bearing support by means of an elastic damper and the stator and bearing support are separated from one another by an air gap adjacent at least part of their facing faces. This design adds damping and isolation, but would be costly to manufacture and also requires additional manufacturing steps to make the motor and precise alignments of parts.
Maughan et al., U.S. Pat. No. 5,365,388 discloses a disk drive that has a stator positioner disposed on a shoulder of a drive shaft housing which is part of a spicule. The positioner secures the stator against movement and references the motor to the spicule which engages and guides the cartridge. An open cell urethane gasket between the printed circuit board and the stator absorbs vibrational forces. This design offers isolation, but does not offer direct damping of the stator, thus its overall effectiveness is limited.
Dunfield et at., U.S. Pat. No. 5,619,389 discloses a spindle motor for rotating at least one disc in a data storage device which includes a base, a shaft, a rotor and a stator. According to the patent a bearing interconnects the rotor with the shaft and allows the rotor to rotate about the shaft. A resilient coupling, such has an O-ring, is compressed between the stator and the base to mechanically isolate the stator from the base and thereby reduce the generation of acoustic noise in the storage device.
The paper appearing in the Journal of the Institution of Engineers, Singapore. Vol. 32 No. 1 February 1992 entitled "Some Aspects of Spindle Motors for Computer Disk Drives" provides an overview of disk drive motor design.
U.S. Pat. No. 5,283,491 entitled, "Air Bearing Motor Assembly for Magnetic Recording Systems" discloses an air bearing motor design. The patent demonstrates that even newer state of the art motors can benefit from an improved method of damping the stator and that prior art methods of damping the stator or isolating the stator have significant performance and manufacturing issues that need to be resolved.
All of the above techniques have drawbacks such as high manufacturing costs, excessive heat retention, significant added mass, difficult assembly procedures for winding wire around the stator core, significant new equipment investment, and high dimensional tolerance design. The aforementioned designs also may not be useful for very high volume motors for disk drives where volumes can be on the order of 1-10 million units a month.